The technical field of this invention is motor speed control and particularly using rotor flux based MRAS (Model Reference Adaptive System) speed observers.
Electronic speed control of AC induction motors is well known in the art. Such AC induction motors are typically driven by selectively switching a DC voltage formed by rectifying an AC power source across the motor phase windings. An electronic controller including a microcontroller or digital signal processor (DSP) controls three phase switching of the DC voltage to produce a pulse width modulated three-phase AC input to the motor. It is feasible to employ speed feedback in these systems. Thus the electronic controller would compare an actual motor speed from a sensor with the command speed. This electronic controller would then continuously modify the drive to the AC induction motor to minimize any difference between the actual motor speed and the commanded motor speed.
A modification of this technique is the subject of this invention. Sensors that generate an electrical indication of motor speed such as tachometers are expensive and unreliable when compared with sensors for measuring electrical quantities such as voltage and current. Thus, speed observers such as rotor flux based model reference adaptive system speed observers are widely used. Rotor flux based model reference adaptive system speed observers employ electrical measurements of currents and voltages in the stator windings of the AC induction motor. The electronic controller employs these measures in a rotor flux estimator. The rotor flux estimate is employed both in generation of the motor drive control signals and in the rotor flux estimation.
A typical prior art system is illustrated in FIGS. 1 and 2. FIG. 1 illustrates the typical AC induction motor drive hardware 100 in block diagram form. AC induction motor drive hardware 100 includes high voltage module 110, electronic control module 120, AC induction motor 130 coupled to high voltage module 110 via three phase AC lines 135 and load 140 coupled to AC induction motor 130. High voltage module 110 includes all electronic parts that must handle high voltages. Alternating current (AC) line power supplied to rectifier/doubler 111 enables production of a direct current (DC) voltage used for induction motor drive. Switching module 113 is shown schematically in FIG. 1. Switching module 113 includes six high voltage semiconductor switches connected in three series pairs between the DC voltage and ground. The junction between each pair of semiconductor switches drives one phase of the three phase inputs 135 to induction motor 130. Predriver module 117 supplies switching signals to the six semiconductor switches. Current shunt module 115 includes a current shunt sensing resistor of resistance Rsense in the ground path of each series pair of semiconductor switches. Signal conditioning module 119 receives the voltage across each of these resistors. The voltage across these sensing resistors corresponds to the current to ground from the corresponding semiconductor switch pair.
Electronic control module 120 provides the control function for AC induction motor drive hardware 100. Electronic control module 120 receives three analog input signals, ADC1, ADC2 and ADC3 from signal conditioning module 119. These three signals correspond to the voltage across the respective current shunt sensing resistor. Electronic control module 120 employs these input signals together with a speed command or other command input (not shown) to produce six switching signals PWM1, PWM2, PWM3, PWM4, PWM5 and PWM6. These six switching signals PWM1, PWM2, PWM3, PWM4, PWM5 and PWM6 are supplied to predriver module 117. Each of the six switching signals PWM1, PWM2, PWM3, PWM4, PWM5 and PWM6 provides control of the ON/OFF state of one of the six semiconductor switches of switching module 113. As known in the art, each of these signals provides a pulse width modulated drive to a corresponding one of the three phase drive lines 135 to induction motor 130.
FIG. 2 illustrates speed estimator algorithm 200 typically used in such rotor flux based model reference adaptive system (MRAS) speed observers. Speed estimator algorithm 200 forms two rotor flux estimates. Reference rotor flux estimator 201 forms rotor flux reference estimate xcexr from stator voltage vector vs and stator current vector is. These two dimensional vectors having direct and quadrature components are derived from the stator voltages and currents of all three phases using the well-known Clarke transformation. It is understood that the rotor flux estimate is based upon the respective inputs for all three phases. Adaptive rotor flux estimator 202 forms adaptive rotor flux estimate xcexa from the stator current is and a motor speed estimate xcfx89. Error calculator 203 determines the difference between the reference rotor flux estimate xcexr and the adaptive rotor flux estimate xcexa. The error signal e supplies a proportional/integral controller 204 which forms the motor speed estimate xcfx89. Motor speed estimate xcfx89 is compared with a commanded speed in a feedback loop. This drives a conventional pulse width modulation algorithm generating the six switching signals PWM1, PWM2, PWM3, PWM4, PWM5 and PWM6.
This rotor flux estimator method offers many advantages over a classic open loop speed observers. It does not involve any open loop integrators or differentiators. It is computationally simple. It does not require a separate slip speed calculation. However, this rotor flux estimator method is prone to stability problems, especially at high speeds.
FIG. 3 illustrates a typical adaptive rotor flux estimator algorithm 202. Gain blocks 301 and 302 receive respective stator currents isd and isq. The output of gain block 301 supplies an additive input of summing junction 305. The output of summing junction 305 supplies low pass filter 309, which produces rotor flux estimate xcexad. Similarly, the output of gain block 303 supplies an additive input of summing junction 307. The output of summing junction 307 supplies low pass filter 311, which produces rotor flux estimate xcexaq. Adaptive rotor flux estimator 202 includes two feedback paths. The rotor flux estimate xcexad supplies one input of multiplier 313. The other input of multiplier 313 receives the motor speed estimate xcfx89. The product output of multiplier 313 supplies a second, additive input of summing junction 307. The rotor flux estimate xcexaq supplies one input of multiplier 315. The other input of multiplier 315 receives the motor speed estimate xcfx89. The product output of multiplier 315 supplies a second, subtractive input of summing junction 305.
Adaptive rotor flux estimator 202 is stable at low speeds where the feedback is near zero. At higher speeds, the cross-coupling becomes significant. The estimator poles become more and more lightly damped at higher speeds. At a sufficiently high speed, the additional phase lag in a digital implementation due to zero order hold reconstruction and processor time delay will cause instability.
A method of AC induction motor control known as rotor flux based model reference adaptive system. Model reference adaptive systems develop two estimates of rotor flux. The reference flux estimate is based on the voltages and currents in the stator windings. The so-called adaptive estimate is based on the stator currents and the measured or estimated operating speed. These two estimates are compared by taking the cross product between the reference and adaptive rotor fluxes and the estimated speed is adjusted by a proportional-integral controller until the estimator outputs agree. In this invention, the upper speed range of the rotor flux-based MRAS speed observer is extended by discretely or continuously modifying the gain/bandwidth parameters of the low-pass filter in the adaptive flux estimator as a function of estimated speed to increase the stability margin.